In general, for MIPI DSI there are three ways to represent the
pipeline for an upstream bridge to find the connected downstream
panel or bridge.

1. Child panel or bridge as a conventional device tree child node.
2. Child panel or bridge as an OF-graph port node.
3. Child panel or bridge as an OF-graph ports node.

There are three different downstream panels or bridges that are
possible to connect an upstream DSI host bridge - DSI Panel,
DSI Bridge, and I2C-Configured DSI bridge.

An example of the downstream panel represented as a child node,

&dsi {
   compatible = "samsung,exynos5433-mipi-dsi";

   ports {
        port@0 {
             reg = <0>;

             dsi_to_mic: endpoint {
                  remote-endpoint = <&mic_to_dsi>;
             };
        };
   };

   panel@0 {
        reg = <0>;
   };
};

An example of the downstream bridge represented as a port node,

&i2c4 {
   bridge@2c {
	compatible = "ti,sn65dsi84";

        ports {
             port@0 {
                  reg = <0>;

                  bridge_in_dsi: endpoint {
                       remote-endpoint = <&dsi_out_bridge>;
                       data-lanes = <1 2>;
                  };
             };

	     port@2 {
	          reg = <2>;

                  bridge_out_panel: endpoint {
                       remote-endpoint = <&panel_out_bridge>;
                  };
             };
        };
   };
};

&dsi {
   compatible = "fsl,imx8mm-mipi-dsim";

   port {
        dsi_in_lcdif: endpoint@0 {
	     reg = <0>;
	     remote-endpoint = <&lcdif_out_dsi>;
	};

	dsi_out_bridge: endpoint@1 {
	     reg = <1>;
	     remote-endpoint = <&bridge_in_dsi>;
	};
   };
};

An example of the downstream bridge represented as a ports node,

&dsi {
   compatible = "fsl,imx8mm-mipi-dsim";

   ports {
	port@0 {
	     reg = <0>;

	     dsi_in_lcdif: endpoint@0 {
		  reg = <0>;
		  remote-endpoint = <&lcdif_out_dsi>;
	     };
	};

	port@1 {
	     reg = <1>;

	     dsi_out_bridge: endpoint {
		  remote-endpoint = <&bridge_in_dsi>;
	     };
	};
};

In, summary it is possible to represent all three downstream slaves
devices using OF-graph port or ports node however only DSI Panel and
DSI Bridge are possible but not possible to represent I2C-Configured
DSI bridge child nodes since I2C-Configure bridges are child of I2C
node, not upstream DSI host bridge and it is must represent them
endpoint port linking.

This indeed means, the OF-graph port or ports representation is
mandatory for I2C-Configured DSI bridges.

This patch tries to add an OF-graph port or ports representation
detection code on top of existing child node detection.

It is possible to replace the entire detection code using existing
drm_of helper drm_of_find_panel_or_bridge but it will break the
Exynos DSI since the pipeline doesn't support OF-graph port or ports
node.

Overall, this patch has a combination of child and OF-graph pipeline
detections in order to support the backward compatibility of Exynos
DSI child node and i.MX8M Mini/Nano/Plus OF-graph port or ports
node pipelines.

This is the first common DSI host bridge driver that needs to support
all possible downstream connection pipeline combinations.
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>